1. Field of the Invention
The present invention relates to a method and apparatus for evaluating the quality of service (QOS) in an ATM multiplexing apparatus in which priority control by buffer reservation (nested threshold) or priority control using a classified priority control scheme (quality control with multiple queues) is performed, and for controlling call admissions and optimizing the priority control on the basis of the evaluation.
Demand for broader-band and larger-capacity communications services has urged the development of the broadband aspects of integrated services digital network (B-ISDN), and asynchronous transfer mode (ATM) has been drawing attention as the switching/transmission system for B-ISDN. To use transmission channels efficiently and transmit large volumes of information economically and safely, the development of an efficient multiplexing transmission system is essential. The present invention is devised to meet such needs.
2. Description of the Related Art
A variety of priority control schemes have been proposed for effectively multiplexing cells with different QOS requests in ATM multiplexing apparatus.
In a priority control scheme called preemptive resume priority, a single common buffer is used, and when high-priority and low-priority cells are stored in the buffer, the high-priority cells always have precedence for transmission (D. Bertsekas, R. G. Gallager, "Data Networks," Prentice-Hall, 1987).
In a priority control scheme called push-out priority control, a single common buffer is used, and when the buffer is full of high-priority and low-priority cells, if a high-priority cell arrives, a low-priority cell in the buffer is discarded. That is, when the buffer overflows, cells are discarded in the order of ascending priority, the lowest priority cell first.
In a priority control scheme called nested threshold, a common buffer is used in which a plurality of threshold values are provided corresponding to a plurality of priority classes, and when the cell length in the buffer has exceeded a certain threshold, any arriving cell having a priority lower than that is discarded and is not stored in the buffer (K. Baba, et al., "On Multiplexing Effect for Bursty Traffic with Priorities Using Layered Modeling Approach in ATM Networks," Trans. IEICE, J76-B-I, pp. 219-230, Mar. 1991, and U.S. Pat. No. 4,953,157).
In a classified priority control scheme (quality control with multiple queues), a plurality of buffers are provided, one for each priority class, and higher priority cells are transmitted at higher priorities. In this control scheme, cell discard occurs in each buffer independently of the other cells. (Y. Takagi, et al., "Cell Transfer Quality Control in an ATM Network," Trans. IEICE, J74-B-I, pp. 180-189, Mar. 1989).
On the other hand, the present inventor proposed, in PCT/JP 93/01243, a technique for evaluating the QOS on the safe side within a practical time by using the characteristic values of incoming traffic, based on the study on the upper and lower bounds for a three-dimensional probability distribution with cell arrival count a.sub.t, cell arrival rate r.sub.t, and cell count (cell length) q.sub.t within a buffer in a unit time as random variables. By applying this technique to a set of call sources having a priority higher than a predetermined priority level, the technique is extended to enable the QOS to be evaluated on the safe side within a practical time from the characteristic values of incoming traffic for a case where priority control is performed by preemptive resume priority for cell transmission and by push-out control for cell discard.
However, with regard to the buffer reservation scheme and the classified priority control scheme, no relationships between parameters defining the input end and the probability distributions of the cell length in the buffer are given in the above references dealing with the other control schemes; therefore, appropriate criteria for the amount of traffic for each priority class that satisfies the desired QOS are not provided, or if a method is available that can calculate the amount, the amount of calculation is so large that practical evaluation is difficult, and further, there is no guarantee that the evaluation can be made on the safe side.
Furthermore, it is desirable that each threshold value in the nested threshold scheme and the size of each buffer in the classified priority control scheme be designed properly to match the distribution of priority classes in actual traffic and be made variable in dynamic fashion with subsequent variations in the traffic characteristics. However, methods of implementing such techniques are not shown in the above references.